The mechanism by which the higher alcohols are formed along with the methanol is not yet exactly known. It is assumed that by the reaction between CO and H.sub.2 --CH.sub.2 OH-- groups are formed on the catalyst surface. These surface groups are transformed by the alkali (A) into the methylate surface groups --CH.sub.2 OA, to which CO is added on with formation of acetate surface groups --CH.sub.2 CO--OA. These groups are reduced with hydrogen to --CH.sub.2 --CH.sub.2 OH surface species. These species form, on the one hand, with hydrogen ethanol, on the other hand, with alkali the ethylate surface groups --CH.sub.2 --CH.sub.2 --OA, which in analogy with the methylate surface groups are transformed into higher alcohols by addition of CO. Probably also aldehydes play a part in the synthesis of the higher alcohols, which would explain the formation of higher aliphatic alcohols with branched carbon chain, as for example isobutanol.
In the catalysts according to the No. DE-OS 30 05 551, the maximum yield of higher alcohols occurs at a potassium content (calculated as K.sub.2 O) of 1.7 wt. %. The preferred range is between 1.7 and 2.5 wt. %, the atomic ratio Cu/Zn being preferably between 0.4 and 1.9.
By the alkalization not only are new active centers created which make the synthesis of higher alcohols possible, but at the same time also the active centers for the methanol formation are blocked. The increasing alkali content in the catalyst thus results in a decreased methanol yield.
It was found, however, that the alkalization of methanol synthesis catalysts containing copper oxide and zinc oxide leads to a faster growth of the Cu crystallites and hence to a gradual deactivation. This disadvantageous effect is the more pronounced the more the methanol synthesis catalyst is alkalized.
It is the object of the invention to make available catalysts of the initially defined kind, by means of which high yields of higher alcohols can be obtained at a relatively low alkali oxide content (at which the growth of the Cu crystallites is not yet pronounced).
It has been found, surprisingly, that not only the alkalization, but also the porosity of the oxidic catalyst precursor plays a crucial role in the synthesis of higher alcohols from CO and H.sub.2. It has been found that the yield of higher alcoholsis generally proportional to the volume of pores with a diameter of less than about 14 nm.